1. Field of the Invention
This invention is concerned with motion sensors for moving magnetically susceptible parts and more particularly with such sensors for detecting motion of magnetically susceptible parts under nonuniform accelerative conditions of the sensor and part.
2. Description of the Prior Art
It is known to use a piezoelectric transducer combined with a magnet to detect motion of the magnetically susceptible part relative to the transducer-magnet combination. Examples of such apparatus are illustrated in U.S. Pat. No. 4,409,548 issued Oct. 11, 1983 to L. R. Focht and U.S. Pat. No. 3,221,213 issued Nov. 30, 1965 to J. R. McKinney. Such magnetically susceptible part motion sensors are satisfactory if the signal produced by the transducer when the part passes the magnet is substantially greater then any noise signal produced by the transducer. Such noise signals are present, for example, when the transducer is bounced around by motion of a vehicle in which the transducer is resident.
In the case of the aforementioned patents, because the magnet can be placed very close to the part, the signal produced by the transducer as the part passes the magnet is far in excess of noise generated due to bouncing. In some situations the magnet and moving magnetically susceptible part can not be placed close together and/or the size of the transducing element is very small such that using the structure of the aforementioned patents the noise signal is in excess of the signals due to motion of the part to be sensed.
U.S. Pat. No. 3,988,620 issued Oct. 26, 1976 to W. T. McDavid is concerned with a pressure-sensitive transducer used for marine seismic exploration systems. There, two transducers are mechanically mated together and electrically connected to cancel the effects of the accelleration through the water. There is no magnet in the structure of U.S. Pat. No. 3,998,620 and the arrangement is in no way suitable for measuring motion of a magnetically susceptible part.